In many applications, it is now common for an internal combustion engine to utilize an exhaust gas turbocharger to improve performance. A turbocharger is a device that uses the exhaust gas of the internal combustion engine to drive a supercharger attached to the engine. The turbocharger usually comprises an exhaust gas turbine and a compressor, both mounted on a common shaft which can be driven by an electric motor. The turbine transfers or switches the electric motor from motor operation to generator operation in the event of excess power in the exhaust gases so to furnish electrical energy to an electric accumulator via a static converter. A control device is fed the actual rotary speed of the turbocharger and the actual voltage of the electric accumulator from respective sensors and an acceleration signal from a transmitter. From the signals provided, the control device forms control signals for the static converter by which the internal combustion engine is accelerated quickly during the motor-operating mode of the electric motor and the power furnished by the electric motor is rectified during the generator-operating mode.
A control device of the type described above is found in the European Patent Office patent document No. EP-A 0079100. The reference discloses that the electrical energy furnished in the generator-operating mode of the electric motor is supplied to a battery acting as the electric accumulator in order to have the energy available again for a subsequent turbocharger acceleration process. Switching the electric motor from motor operation to generator operation and vice versa is accomplished by means of a mechanical or electronic switching relay as a function of the turbocharger speed, the battery voltage and the acceleration signal. During the generator-operating mode, energy is supplied to the battery via an uncontrolled rectifier bridge. During the motor-operating mode, energy from the battery is fed to the stator winding of the electric motor via an inverter controlled by the control device. The rectifier bridge and the inverter may be replaced by a bi-directional static conterter.
Although the aforedescribed control device makes quick acceleration of the internal combustion engine possible, low-smoke combustion and low fuel consumption, which are both desirable and advantageous, are not obtainable for the following reasons. The amount of fuel supplied to an internal combustion engine is directly proportional to the signal emitted by an acceleration transmitter. For smoke-free combustion, i.e. combustion at the optimum fuel/air ratio, the supplied amount of fuel requires a closely defined amount of combustion air. The aforesdescribed control device can produce exhaust gas containing excess energy, however, only at the expense of excess fuel intake to the internal combustion engine. This automatically leads to a fuel/air ratio unfavorable for combustion and, hence, to exhaust gas turbidity. Neither this excess fuel supply nor the resultant fuel/air ratio for the generation of otherwise useable excess exhaust gas energy are desirable. Disadvantageously, the higher amount of fuel also increases the operating costs. Lastly, the worsened exhaust gas composition is usually impermissible, especially from the aspect of the exhaust gas ordinances in force.
Another control device for an internal combustion engine with an exhaust gas turbocharger is described in the German Pat. DE-A No. 22 06 450. The turbocharger comprises an exhaust gas turbine and a compressor mounted on a common shaft. The shaft can be driven by an electric motor controlled so that, in the event of excess power in the exhaust gases, the electric motor is switched from motor operation to generator operation and electrical energy is supplied to a network fed by a supply source. The decisive evaluation and comparison criterion disclosed therein is that the electric motor keeps the speed of the disengaged turbocharger constant in the entire load range of the internal combustion engine by means of an appropriate control device. Thus, the turbocharger compressor always furnishes, regardless of the momentary loading of the internal combustion engine, an output of constant level with the result that the internal combustion engine is always offered a constant amount of air of constant charging air pressure. Consequently, if the internal combustion engine is to be accelerated from idling, for instance, the excess air prevailing in the lower and medium partial load range will force an increased fuel intake to ensure the fuel/air ratio required in the internal combustion engine.
According to another disclosed criterion of the German patent, the control device is designed so that, in the event of excess pwoer at the exhaust gas turbine, the electric motor is switchable to generator operation and electrical energy can then be supplied to the network fed by the supply source. However, assuming that the electric motor in its motor-operating mode drives the turbocharger at constant speed (i.e., for constant charging air pressure and constant amount of charging air) over the entire load range of the internal combustion engine, such a power excess, capable of causing the electric motor to switch from motor to generator operation, in the exhaust gases supplied to the exhaust gas turbine for conversion into electrical energy can be achieved only by an increased fuel intake.
Therefore, it is an object of the invention to improve a control device of the kind defined at the outset so that, in operating an internal combustion engine with less fuel consumption than in previously known arrangements, better acceleration of the internal combustion engine in the partial load range, particularly for starting and in the lower part of the load range, as well as a most favorable conversion of excess exhaust gas energy into electrical energy is possible. It is a further object of the invention that the electrical energy from such a conversion is not to be supplied to a battery, but to a consumer load to be provided and present in the surroundings of the internal combustion engine and whose specific data must also be taken into account in the design of the control device.